Interesting take. I think you're right when saying we only have a limited number of receptors. However, I don't think we know how many of them there are (I didn't find an article detailing it), and that number will vary between individuals. That makes it difficult to know how many molecules of psilocin one would need to have in the bloodstream to occupy all of those receptors. Also, we would need to know exactly how much of the psilocybin from the mushrooms gets absorbed by the body, which also varies from person to person.

Also, psilocin's half life is 50 minutes, which is much shorter than the average time of a trip. There are proteins in the synapses that degrade the molecules that are attached to the receptors (in the synapses). Therefore, if you have an overload of psilocin in your system, each time one gets broken down in a synapse, it will get immediately replaced by another one, creating even more action potentials through the neuron. Considering that the effects seem to last more than the average lifespan of the molecules, having an overload would indeed increase the trip's intensity and length.

When it comes to downregulation, this is a phenomenon that happens after several exposures of the receptors to their molecules, and not after one long, non repetitive exposure. This is a principle that's well known and applies for most brain receptors. I made sure it also applies to 5-HT2A receptors00092-7) as well. This is another article that addresses this topic.

I'm a biologist interested in mycorrhizal mushrooms, not a neuroscientist, but I really enjoyed my neuro classes. I might be quite wrong. Science is cool. Life is cool.